Robotic bat wing reveals flight secrets of bats

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The lesser dog-nosed bat and the robotic bat wing

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Recently, we've seen a robotic ostrich. Now, there’s a robot bat – or at least, part of one. Joseph Bahlman, a graduate student at Brown University, with the help of Professors Kenneth Breuer and Sharon Swartz, has developed a robotic bat wing that mimics the ligaments, skin and structural supports of the real thing. The purpose of the motorized plastic bat is to gain a better understanding of how bats are engineered and fly.

Bats are much more closely related to us than birds, but they tend to be a bit mysterious in a way that birds aren't. It isn't simply that they fly around at night and are associated with overdressed Transylvanian counts with strange diets. They are also the only flying mammals, and exactly how they remain airborne isn't entirely understood.

Bats aren’t like airplanes. You can’t just stick one in a wind tunnel and they won’t play along if you try. “We can’t ask a bat to flap at a frequency of eight hertz then raise it to nine hertz so we can see what difference that makes,” said Bahlman. “They don’t really cooperate that way.”

They’re also very complex. The wings stretch the length of the bat’s body and are supported by the arms and hands – indeed, the wing is practically all hand. Over this framework is a remarkably elastic skin that can stretch 400 percent without tearing. This complexity works against researchers, since it's very difficult to single out any particular sets of factors for study.

These points make a robot very attractive because it is not only cooperative, but can be simplified. Or rather, it is simplified, since the technology to exactly duplicate a bat wing doesn't exist yet. The Brown University robotic bat was designed and constructed by Bahlman with the help of Breuer, an engineer and Swartz, a biologist. The eight-inch robot is based on the lesser dog-faced bat. It has 3D-printed plastic “bones” and a silicone elastomer skin. The seven joints are actuated by three servo motors pulling cables that act like tendons.

The robot is not intended for flight, but for wind tunnel tests. There’s a transducer in it that measures the forces on the wing as it flaps in the tunnel. By measuring these forces, the researchers can learn a great deal about how a bat’s wing works in flight.

“We can answer questions like, ‘Does increasing wing beat frequency improve lift and what’s the energetic cost of doing that?’” Bahlman said. “We can directly measure the relationship between these kinematic parameters, aerodynamic forces, and energetics.”

For example, the robotic wing helps to understand the role of wing folding. Bats and birds do this when flapping to save energy on the upstroke, but testing with the robotic wing also shows that the folding changes the wing's aerodynamic qualities. An upward moving wing should force the bat down, but the nature of the folding decreases this force by 50 percent.

It isn’t just wind tunnel tests that yield knowledge. Building the simplified robot wing revealed new insights. “We learned a lot about how bats work from trying to duplicate them and having things go wrong,” said Bahlman. Testing showed that the robotic wing would fail in surprising ways, such as the elbow breaking or the skin tearing. Repairing these faults suggested ways in which the tendons, ligaments and muscles reinforced these weak spots and kept them from failing in real life.

The robotic bat wing is not just an exercise in idle curiosity about bats. Bahlman contends that it also has practical applications in developing very small aircraft, such as Micro Air Vehicles, that could fly like a bat.

As to the Brown robotic bat, Bahlman said, “The next step is to start playing with the materials. We’d like to try different wing materials, different amounts of flexibility on the bones, looking to see if there are beneficial trade offs in these material properties.”

The results of the study were published in Bioinspiration and Biomimetics.

The video below shows the robotic bat wing in action.

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